The invention relates to a method of manufacturing a contact construction structure consisting of a base metal or base metal alloy structure provided with a corrosion resistant contact surface layer of silver or a silver alloy or of palladium or a palladium alloy.
Contact construction structures with contact surface layers of silver or palladium or silver or palladium alloys have the disadvantage that a thin but highly resistant corrosion film is formed on their surfaces quite rapidly which corrosion film results in a relatively high cross-contact current flow resistance. In spite of the otherwise excellent properties of these contact construction materials, other contact construction materials with relatively thick layers of gold as contact surfaces are still widely used. Such heavy application of gold however causes the contact construction material structures to be relatively expensive, and it is therefore the object of the present invention to provide a contact construction material structure which utilizes the advantageous contact materials mentioned above in such a manner that they remain essentially free of undesirable corrosion films.
It is known to apply to contact surfaces gold coatings of about 0.5 to 1.0.mu. thickness and such coatings will in fact retard the formation of objectionable corrosion films as shown in the article "Diffundierte Goldschichten fur schaltende Kontakte" co-authored by the inventor and printed in Edition 1, 1987 of the Magazin METALL, METALL-Verlag GmbH, 1000 Berlin 33, Germany. Corrosion films however will rapidly develop on the untreated surface areas which then will have substantially increased resistivity.
It would, of course, be possible to provide the contact surfaces with a relatively heavy gold coating but this would be quite uneconomical because of the relatively heavy use of goldlike with the contact structures mentioned before. It is also known to provide gold or silver coatings of greater thickness than the 0.5-1.mu. mentioned before and of smaller thickness than the heavy gold coatings utilized and proven effective with present contact structures, that is, coatings of 5.mu. to 20.mu. which consist of gold or silver or alloys thereof. They have been found to be quite suitable and durable for many applications as they prevent the formation of the corrosion films described above and are economically still quite bearable.
It is further known that pure palladium contact members, with which this problem, though somewhat less pronounced, also occurs, can be plated mechanically with a thick gold cladding (German OS 31 43549) which cladding is rolled onto the contact members in several steps so as to achieve the desired dimensions. Intermittently, the contact structures are exposed for short periods of time to temperatures of about 760.degree. C. As a result of such temperature treatment the gold diffuses into the base palladium such that a corrosion inhibiting metal alloy layer is formed which has a gold content that decreases with distance from the surface. The manufacture of such corrosion resistant contact members however is also quite involved because of the step of plating the contact members with a gold layer.